Presently the largest single application area for photovoltaic modules in the United States is for residential power in relatively small systems installed on residential roofs. Although the electrical output of the average system size is increasing, with 1 kilowatt to 2 kilowatt systems becoming common, the present known modules are still relatively small and largely insufficient.
Nearly all high-power photovoltaic modules (&gt;40 watts) are manufactured with the photovoltaic cells encased using glass as the top surface or the back surface or both. Usually, a picture flame-style metal frame is attached to the edges of the glass as a containment, and to lend some support to the unit, and to provide a means to fasten the modules to larger frames.
In order to install such prior art modules to a roof, the modules are commonly mounted to a larger frame, e.g., the roof frame, which is in turn directly fastened to the roof on top of the finished roof surface. Alternatively, features of the roof frame are fit into roof jacks which are fastened to the roof, again on top of the finished roof surface.
There are several disadvantages in the known prior art.
Modules which are manufactured using glass as a substrate or superstrate or as both surfaces have a high weight per unit area and are relatively fragile. Although large modules would be desirable in order to minimize handling, wiring, and other installation parameters, the larger the module area, the heavier per unit area and the more fragile the modules become, and the greater the difficulty in handling. Experience with designing and installing a residential photovoltaic system using one such large, commercially available module model having 12.5 square feet of area each has shown that the weight per unit area of these modules is on the order of 3.4 pounds per square foot, and they must be handled with extreme care. With four modules bolted to a wooden frame in a sub-array, the sub-arrays weigh nearly 190 pounds and require mechanized lifting equipment to position them on a roof. A significant fraction of such modules have been found to develop debilitating cracks between the time they are shipped and the end of the first year of operation.
The standard picture frame-stile or other external flaming for the module adds considerable cost in material and labor to the manufacture cost of the finished photovoltaic module.
The design and fabrication of the roof flames and roof jacks is usually left to the installer and involve considerable labor and material cost. Commercially available, pre-made roof frames are similarly expensive.
Significant labor is required to fasten the prior-art modules to the roof frames, and to lay out and fasten the roof frames to the roof; or to lay out and fasten the roof jacks to the roof and the roof flames to the roof jacks.
Because of the construction of such prior art modules, in mounting them spaces usually must be left between modules. This provides an opportunity for debris to collect on the roof. Sometimes spaces must be left which are large enough to permit the installer to fasten between the modules or sub-arrays. The appearance of the module frames and spaces between modules and between sub-arrays is regarded by many homeowners as aesthetically unsightly on a finished roof.
There is also an incentive to reduce installed cost through roof-integrated installations--that is, replacing certain portions of the roof construction with the photovoltaic modules themselves. This root-integration effort has been attempted through approaches similar to glazing methods for skylights, but such glazing methods have resulted in added costs which for the most part are higher than the amounts they save in replacing portions of the roof, therefore defeating their purpose.
Another drawback of most previous roof-integrated photovoltaic module designs and system designs is that they have not allowed for the free flow of cooling air beneath the photovoltaic cells. Higher cell operating temperatures result in lower power output and/or lower voltage from photovoltaic cells. In systems installed on an insulated roof, for example, in typical New Jersey weather conditions, lack of air cooling on the back surface of the photovoltaic cells can result in 40.degree. C. higher cell temperatures compared to systems with back surface air flow, according to calculations based on heat transfer simulations. In crystalline silicon cells, for instance, this temperature increase would decrease power output by about 18%, and in certain instances could lower the module voltage to a level which could not effectively charge batteries. In amorphous silicon cells, as another example, the increase in temperature could result in slightly lower power output. It could also significantly decrease the module voltage, forcing a trade-off between module power output and module design voltage for effective battery charging. If a higher design voltage is chosen to ensure battery charging in hot weather for this reason, the result may be a loss of power for the module in average and colder weather, due to a mismatch between battery voltage and optimum module voltage.
Another problem with higher operating temperatures due to lack of cooling on the back surface of the photovoltaic unit is that most photovoltaic cells and modules have been developed with voltages appropriate for battery charging in systems with back surface cooling. Therefore, raising the maximum operating temperature of such prior art modules may necessitate the redesign of some photovoltaic cells, or the use of odd numbers of them in series, in order to maintain proper voltages.
Other prior art roof integrated module and system designs have attempted to avoid this problem of cooling the cells by utilizing glazing methods for installing the modules over roof framing members with the back surface of the modules open to an unheated attic space. However, this method is not applicable to insulated roofs. Moreover, unheated attic spaces will have a significantly higher air temperature than the outside air, due to solar heat input from the roof, and there will be less air movement underneath the roof compared to the outside air. This will decrease the degree to which the back of the modules can be cooled. If a fan moving a large amount of air through the attic is installed to cool the modules during the summer, additional expense and parasitic power use result.
Accordingly, a need exists for an improved photovoltaic module which has the capability of providing for simplified installation, ease of handling including low weight, ruggedness, high strength, and stiffness, very large module formats, and roof integration without sacrificing superior back surface cooling.